1. Field of the Invention
This invention relates generally to vehicle suspension systems. More particularly, this invention relates to a rear suspension swingarm assembly for supporting a rear wheel of a motorcycle.
2. Description of the Related Art
Motorcycles generally include an engine supported within a cradle-type frame assembly. A typical arrangement includes front suspension fork assembly supported by the frame for rotation about a steering axis and supporting a front wheel of the motorcycle at its lower end. A handlebar assembly is mounted to the suspension fork to permit steering of front wheel of the motorcycle. An elongated, straddle-type seat assembly is typically supported above the frame. A rear wheel of the motorcycle is commonly supported by a suspension swingarm assembly, which is pivotally supported by, and extends in a rearward direction from, a lower portion of the frame. Thus, both the front and rear wheels are typically suspended for limited, controlled movement along a suspension travel path relative to the frame.
A common swingarm assembly includes a connecting portion, which is typically connected to the motorcycle frame and to which one end of a suspension member, such as a hydraulic shock absorber, is mounted. One or more tubular arms typically extend in a rearward direction from the connecting portion and a rearward end of the arm supports the rear wheel of the motorcycle. The arm(s) often have a substantially rectangular cross-sectional shape.
A typical method of producing an arm portion is called swaging, which begins with a tubular workpiece that is slightly larger than the final product and roughly formed in the shape of the final product. The workpiece is then subjected to a swaging process, wherein an appropriately sized and shaped die is placed within the workpiece and a series of blows are rapidly applied to the external surface of the workpiece, by a hammer, die or other hard object, to mechanically deform the workpiece to conform to the outer surface of the die. Once the workpiece has been deformed, the die is removed through the larger opening at the end opposite the reduced portion that has been formed by swaging. In this manner, the arm is reduced in size in desired regions.
One limitation to swaging is that any die that is inserted into the workpiece must be removed. Accordingly, it is not possible to use this process to reduce the cross-section at both ends of a single member while maintaining an enlarged center cross-section. In addition, swaging involves applying blows to the arm to cause the desired deformation. The blows generally cause scarring, of the outer surface of the arm. Accordingly, additional process steps are required to remove the scarring such that an acceptable surface finish can be provided on the arm.
The swingarm assembly of a motorcycle supports the rear wheel and is subjected to large impact forces during motorcycle operation. For instance, large impact forces occur during high-speed turns, when operating over rough terrain or when landing from jumps. The forces also are not limited to a single direction. Rather, the forces encountered vary around all of the directions. Thus, the swingarm is likely to experience large vertical, lateral and torsional forces. Thus, the swingarm must have sufficient strength and stiffness to withstand these varied forces.
One way to increase the strength and stiffness of the swingarm assembly is to increase the overall sizing of a uniformly sized swingarm (e.g., lengthen the outer dimensions of height and/or width). As will be appreciated, the forces and localized stress values vary over the length of the swingarm. Thus, increasing the size involves making the entire uniformly sized swingarm as large as necessary to accommodate the highest localized stresses. Such a design overcompensates for the stress likely to be felt over the entire length of the swingarm. Such a design, therefore, is unnecessarily heavy due to the extra material.
Accordingly, the arm may be produced such that the cross-sectional dimensions of the arm can be reduced from one end, or portion, to the other end. By tapering the ends, a reduced cross-section can be placed in areas subjected to lower bending forces and respective stress. It is desirable to produce a unitary member for use on a swingarm having a greater width in an intermediate portion than the width of either of the end portions of the arm.
With current manufacturing methods, producing such a unitary member for use on a swingarm generally cannot be achieved. This is largely because the swaging die must be removed from within the arm at the completion of the swaging process. Thus, an opening at one end of the member must be at least equal to the largest cross-sectional dimension of the die (i.e., the larger end of a swaged surface) in order to permit the removal of the die. As a result, arms produced by conventional methods cannot be tapered at both ends and, therefore, are heavier than what is necessary to provide sufficient strength and stiffness.
While two tapered portions could be formed and welded together, such a construction increases the likelihood of breakage adjacent to the weld. The increase in the likelihood of breakage is due to the effects of the weld and the associated heat. Thus, while such a two piece construction is feasible, it is less desired than a unitary construction not requiring such a joint between two separate components.
Accordingly, a preferred arm portion for a swingarm assembly is produced by a hydroforming method, wherein a tubular workpiece is placed within a die. The die generally defines a die cavity sized and shaped substantially identical to a desired size and shape of the finished arm. Fluid is introduced within the tubular workpiece and pressurized to a sufficient level such that the workpiece expands to conform to the surface of the die cavity.
One aspect of the present invention involves an off-road motorcycle comprising a frame and a front suspension assembly rotatably supported by the frame. The front suspension assembly rotatably supports a front wheel of the motorcycle. A rear swingarm assembly is pivotably supported by the frame and rotatably supports a rear wheel of the motorcycle. An internal combustion engine is supported by the frame and is configured to drive the rear wheel. The swingarm assembly includes at least one elongated, tubular arm extending along a side of the rear wheel in a direction substantially aligned with a vertical, central plane of the motorcycle. The arm has an intermediate portion defining a first width in a direction substantially normal to the central plane. A forward end defines a second width and a rearward end defines a third width of the arm. The first width is greater than both of the second width and the third width.
Another aspect of the present invention involves a rear suspension swingarm assembly for an off-road motorcycle. The swingarm assembly is configured to be pivotably supported by a frame of a motorcycle and rotatably support a rear wheel of the motorcycle. The swingarm assembly includes a connecting portion defining a pivot axis of the swingarm assembly relative to the frame of the motorcycle. The swingarm assembly additionally includes a first elongated, tubular arm and a second elongated, tubular arm. The first and second arms being fixed to the connecting portion and extending in a rearward direction therefrom. Each of the first and second arms has an intermediate portion defining a first width, a forward end defining a second width and a rearward end defining a third width of the arms. The first width is greater than both of the second width and the third width.
A further aspect of the present invention involves an off-road motorcycle comprising a frame, a front suspension assembly rotatably supported by the frame and rotatably supporting a front wheel and a rear swingarm assembly pivotably supported by the frame and rotatably supporting a rear wheel. An internal combustion engine is supported by the frame and is drivingly coupled to the rear wheel. The swingarm assembly comprises at least one hydroformed-arm extending along a side of the rear wheel in a direction substantially aligned with a vertical, central plane of the motorcycle. The arm has an intermediate portion defining a first width in a direction substantially normal to the central plane, a forward end defining a second width and a rearward end defining a third width of the arm with the first width being greater than both of the second width and the third width.
Another aspect of the present invention involves a motorcycle comprising a frame, an engine mounted to the frame, a rear swingarm pivotally connected to the frame and a rear wheel supported by the swingarm and driven by the engine. The swingarm has an arm portion that is unitarily formed and that has an enlarged central portion relative to the ends. The arm portion is formed by providing an elongated tubular workpiece, providing a die having a first portion and a second portion, the first portion and second portion of the die cooperating to define a die cavity sized and shaped substantially identical to a desired final shape of an outer surface of the arm, placing the workpiece within the die cavity and closing the die, introducing a fluid into an internal space of the workpiece, and pressurizing the fluid to a sufficient level such that the workpiece conforms substantially to the size and shape of the die cavity.